<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<p>计算器</p>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
  结果:<span></span>
</div>

<script>
    let i1 = document.querySelector("#i1")
  let i2 = document.querySelector("#i2")
  let s = document.querySelector("span")
  function f(x) {
    if (isNaN(i1.value) ||isNaN(i2.value)){
      s.innerText = "输入错误!";
      return;//结束方法
    }
    switch (x){
        case 1://加
    }
    s.innerText= i1.value*1+i2.value*1;
  }

</script>

</body>
</html>